Phonon density of states in ${\text{NdFeAsO}}_{1-x}{\text{F}}_x$

We report measurements of the phonon density of states in iron oxypnictide superconductors by inelastic x-ray scattering. Good agreement with ab initio calculations that do not take into account strong electronic correlations is found, and an unpredicted softening of phonon branches under F doping of these compounds is observed. Raman-scattering experiments lead us to conclude that this softening is not related to zone-center phonons, and consequently imply an important softening of the relevant phonon branches at finite-momentum transfer $Q$.

Figure 1

(Color online) (a) Measured (dots) and calculated (solid line) synchrotron x-ray powder-diffraction profiles $\left(\lambda =0.7\text{Å}\right)$ for NdFeAsO at room temperature. The lower solid lines show the difference profiles and the tick marks show the reflection positions. (b) Measured (dots) and calculated (solid line) synchrotron x-ray powder-diffraction profiles $\left(\lambda =0.7\text{Å}\right)$ for ${\text{NdFeAsO}}_{0.85}{\text{F}}_{0.15}$ at room temperature. The lower solid lines show the difference profiles and the tick marks show the reflection positions (from top to bottom: from ${\text{NdFeAsO}}_{0.85}{\text{F}}_{0.15}$ to NdOF).

Figure 2

(Color online) Comparison of the generalized phonon density of states of NdFeAsO (upper panel) with the results of a density-functional perturbation-theory calculation (lower panel; see details in the text). The dashed curve in the upper panel corresponds to the quantity $I_{\text{inelastic}}\left(\omega \right)\left\{\omega /\left[1+n\left(\omega ,T\right)\right]\right\}$, before multiphonon contribution subtraction. The areas have been normalized to 1. The inset of the upper panel shows the IXS measurement prior to the subtraction of the elastic line.

Figure 3

(Color online) Evolution of the PDOS of NdFeAsO under F:O substitution. The areas of both spectra have been normalized to unity. The lowest blue trace with hatched area shows the difference between the two spectra.

Figure 4

(Color online) Raman-scattering spectra of NdFeAsO (black squares) and ${\text{NdFeAsO}}_{0.85}{\text{F}}_{0.15}$ (orange circles). The dotted lines are the individual fits for each of the three phonon lines and the solid line is the sum of the fitted lines.